Ever since the beginning of the space age, communication satellites have been an important application of space technology. The first communication satellite was Telstar. At the time, it was an extraordinary technological achievement. It was designed, built and operated by Bell Telephone Laboratories, Holmdel, N.J., USA.
Communication satellites receive and transmit radio signals from and to the surface of the Earth for the purpose of providing communication services. With Telstar, which was the first and only communication satellite of its time, it was not possible to provide uninterrupted communication services to every location on the surface of the Earth. Only the few locations that happened to be within view of the satellite, at any given time, could transmit and/or receive radio signals to/from the satellite. In more modern communication-satellite systems, it is often desirable that every place on Earth be provided communication services at all times, a capability usually referred to as universal coverage. Furthermore, there are places and locations on Earth that require more communication capacity than others. For example, cities and other densely populated locations can be expected to require more communication capacity than locations in deserted areas.
For the purposes of this disclosure and the appended claims, the terms “place” and “location” have somewhat different meanings. Both terms refer to a portion of the surface of the Earth at a known position (latitude and longitude) relative to the Earth itself. However, the term “place” is understood to refer to something small enough to be effectively equivalent to a single point on the surface of the Earth, while a location can be of any size. For example, a small island, or a village, or a ship at sea can be referred to as a “place” but also as a “location”; in contrast, most countries in the world are too large to be referred to as a “place” and can only be referred to as a “location”. It will be clear to those skilled in the art, after reading this disclosure, when a “location” can also be referred to as a “place”.
Among the examples cited in the previous paragraph, the ship at sea underscores the fact that a “location” does not have to be at a fixed, immutable position on the surface of the Earth. Indeed, satellite-communication systems are particularly useful for providing communication services to non-fixed objects such as ships, aircraft, buses, automobiles, etc. Many techniques are well known in the art for measuring the position of a non-fixed object. For example, the Global Positioning System (GPS) can be used to measure the position of a ship at sea or other non-fixed object. If the position of an object on the surface of the Earth is known, it can be regarded as a “location” in accordance with the definition of the previous paragraph.
The goal of universal coverage via communication satellites can be accomplished with a satellite system based on low earth orbits (LEO). For the purposes of this disclosure and the appended claims, a satellite orbit shall be regarded as a LEO orbit if the satellite is always within 2,000 km of the surface of the Earth at all points in the orbit. An equivalent definition is that the altitude of the satellite above the surface of the Earth must not exceed 2,000 km.
A LEO orbit is called a “polar” orbit if it passes above or nearly above both poles. For the purposes of this disclosure and the appended claims, a LEO polar orbit is a LEO orbit whose ground track intersects both the Arctic and the Antarctic circles on the surface of the Earth. The polar caps encircled by the Arctic and Antarctic circles, respectively, are referred to as the “polar regions”.
FIG. 1 depicts a possible LEO polar orbit 150 for a communication satellite, depicted as LEO satellite 140. FIG. 1 shows an outline of planet Earth 110, with outlines of continental masses clearly delineated. The positions of the North Pole 120 and the South Pole 130 are indicated by a straight line that represents the axis of rotation of the Earth. The orbit passes exactly above the two poles. The satellite travels along the orbit in the direction of motion 101 indicated by the arrow. With an orbit as depicted in FIG. 1, the satellite takes almost two hours to complete a full orbit.
FIG. 2 presents a more detailed depiction of the satellite and its relationship to the surface of the Earth below it. (In this detailed figure and in some of the subsequent figures, continental outlines on the surface of the Earth have been omitted to avoid visual clutter). The LEO satellite 140 is equipped with one or more radio antennas, depicted as radio antenna 210. The antennas transmit one or more radio signals toward the surface of the Earth 110. Such transmissions are shown in the figure as antenna beam 220. The radio transmissions can be received by Earth terminals that are located on the surface of the Earth within a coverage area depicted as coverage area 230. The satellite is also capable of receiving radio signals transmitted by the Earth terminals. For communication satellites, the radio signals can be used to support communication channels, thus providing bi-directional communication services to those Earth terminals. Conversely, Earth terminals that are located outside of the coverage area cannot receive strong-enough signals from the satellite, and their transmitted signals will not be received with adequate strength by the satellite.
For the purposes of this disclosure and the appended claims, the term “Earth terminal” refers to communication terminals operated by end users of the communication services provided by a communication-satellite system. In many such systems, the communication services provide connectivity with Earth-based networks such as the Internet. Therefore, satellites in such systems typically also have antennas for relaying communication channels to relay stations on the Earth that are connected with Earth-based networks. Such relay stations are typically operated by the operator of the communication-satellite system or its affiliates, and should not be regarded as “Earth terminals” for the purposes of this disclosure and the appended claims.
Radio antennas used for communication channels with Earth terminals are referred to as communication antennas in contrast to antennas used, for example, to support control channels or for communicating with relay stations. Earth terminals are devices located on or near the surface of the Earth (including, for example, on aircraft or ships at sea) that are capable of transmitting and receiving radio signals for communicating with communication satellites through the satellites' communication antennas.
For the purposes of this disclosure and the appended claims, the “coverage area” of a satellite is the portion of the surface of the Earth wherein Earth terminals can access communication services via the satellite through one or more of the satellite's communication antennas. The coverage area of a satellite moves on the surface of the Earth together with the satellite, as the satellite travels along its orbit. Typically, a satellite's coverage area is centered around the subsatellite point, depicted in FIG. 2 as subsatellite point 245. The subsatellite point is the point, on the surface of the Earth, nearest the satellite. From this point, the satellite appears exactly overhead, at the zenith. As the satellite travels along its orbit, the subsatellite point moves along with it. The path traced by the subsatellite point is known as the “ground track” traced by the satellite.
Although the coverage area is shown in FIG. 1 as having a circular shape, other shapes are also possible.
FIG. 3 depicts how a rectangular or quasi-rectangular shape for coverage areas can be advantageous. A quasi-rectangular shape allows efficient coverage of the surface of the Earth with no areas left uncovered and with only a modest amount of overlap between adjacent coverage areas. The figure shows quasi-rectangular coverage areas 301 through 306 arranged so as to provide such complete coverage with a modest amount of overlap.
FIG. 4 illustrates the relationship between a satellite's orbit and the satellite's ground track. LEO satellite 440 orbits the Earth in LEO polar orbit 450. As the satellite travels along its orbit, the subsatellite point 445 traces a path on the surface of the Earth. The path is depicted in FIG. 4 as ground track 447. Orbit 450 is a polar orbit in accordance with the definition provided earlier because ground track 447 passes well within the two arctic circles. In particular the orbital inclination of orbit 450, as depicted, is approximately 80°
In FIG. 4, the satellite's orbit is circular, and, accordingly, the satellite's ground track 447 is depicted as a great circle on the surface of the Earth. However, as already noted, the satellite needs almost two hours to complete a full orbit. During such period of time, the Earth rotates by almost 30°. Therefore, the depiction of continental outlines and gridlines on the surface of the Earth in FIG. 4 should be interpreted as just a snapshot of the Earth's position at a single point in time during that period of time. As the subsatellite point travels along the ground track, the Earth rotates at a steady rate such that the actual path traced by the subsatellite point on the surface of the Earth will not be a circle. When the satellite completes a full orbit and returns to the same point in the orbit, the subsatellite point will not be at the same place on the surface of the Earth.
In general, the subsatellite point will never return to the same exact place on the surface of the Earth unless the period of the orbit happens to have been chosen on purpose to achieve such a result. For example, the orbital period of GPS satellites was chosen such that the subsatellite point retraces the same ground track after about two orbits. To achieve this result for GPS satellites, the orbital period was carefully chosen to be almost the same as one half of a sidereal day. Its exact value was devised such that, even in the presence of orbital precession caused by tides and by the Earth's flattening at the poles, the GPS satellites retrace the same ground track after two full orbits.
In this figure and in the other figures in this disclosure where continental outlines and/or gridlines are depicted, it will be understood that such outlines and gridlines represent a snapshot of the Earth's position at a particular point in time, and that the Earth is actually rotating at all times. In such figures, patterns depicted on the surface of the Earth should be understood to be what they would be if the Earth were not rotating. It will be clear to those skilled in the art, after reading this disclosure, how to modify those patterns, if desired, to account for the Earth's rotation. The patterns depicted in this disclosure are best suited for illustrating the present invention and its embodiments.
In FIG. 4, the coverage area of satellite 440 is not depicted explicitly. However, from the depiction of FIG. 2 it is clear that only a small portion of the surface of the Earth below the satellite will enjoy communication services through the satellite at any given time. To achieve universal coverage, multiple satellites are required.
FIG. 5 shows how multiple satellites in the same orbit (i.e., co-orbiting) can provide continuous uninterrupted coverage to locations below the orbit (i.e., along and near the ground track). LEO polar orbit 150 is a circle, and twenty-four satellites 540 orbit the Earth in orbit 150. (In the figure, to avoid visual clutter, only five of the twenty-four satellites 540 are labeled explicitly.) The satellites are depicted as black dots. They are spaced uniformly along the orbit and, because the orbit is circular, they all move at the same speed at all times, such that the spacing between satellites remains constant. Each satellite provides communication services to a coverage area centered around its subsatellite point. Accordingly, in FIG. 5 there are twenty-four coverage areas 530. (In the figure, to avoid visual clutter, only four of the twenty-four coverage areas 530 are labeled explicitly.) It is advantageous if the coverage areas of the satellites have a quasi-rectangular shape as illustrated in FIG. 3 because adjacent coverage areas can provide continuous coverage with a modest amount of overlap.
FIG. 6 shows the twenty-four co-orbiting satellites 540 from a different viewpoint. The figure also shows the shape of the combined coverage provided by the satellites. It is depicted as coverage strip 647-1. It has the shape of a ribbon (hence the name “strip”) that encircles the Earth, with the satellite ground track tracing the center line of the ribbon. (The individual coverage areas 530 are not shown explicitly). It is clear from FIG. 6 that coverage strip 647-1 covers only a portion of the surface of the Earth; therefore, in order to provide universal coverage, more satellites in more orbits are needed.
FIG. 7 depicts two distinct satellite orbits wherein both orbits are circular LEO polar orbits with the same shape, altitude and inclination. Kepler's laws dictate that the two orbits must intersect one another at two points. One of the intersection points is visible in the figure as intersection point 751 located near the North Pole. The other intersection point is near the South Pole and is hidden from view.
FIG. 8 depicts the coverage strips corresponding to the two orbits of FIG. 7. Coverage strip 647-1 corresponds to orbit 450 and is depicted with vertical hatching; coverage strip 647-2 corresponds to orbit 750 and is depicted with horizontal hatching. The angle between the two orbits was deliberately chosen such that the two coverage strips barely touch one another as they cross the Earth's Equator 810. However, at other latitudes, as the strips approach the polar regions, there is more and more overlap between the two strips. The area of overlap is depicted as area of overlap 860 with both vertical and horizontal hatching.
It is clear from the depiction of FIG. 8 that the addition of a second orbit expands overall coverage, but the covered area is not doubled because there is substantial overlap between the two coverage strips. The presence of overlap can be regarded as a waste of resources because, in the area of overlap, there are two satellites available at all times to provide redundant coverage. One might argue that such redundant coverage actually provides an opportunity to offer greater communication capacity to Earth terminals located in the area of overlap. Indeed, an Earth terminal located in that area could communicate with both satellites and thus enjoy double capacity, compared to communicating with only one satellite. Equivalently, two distinct Earth terminals in that area could communicate with two distinct satellites, such that each terminal enjoys the full capacity of one satellite, instead of having to share such capacity with the other terminal.
Unfortunately, such enhanced capacity is not as useful as might seem. This is so because of the Earth's rotation. As already noted above, the Earth is constantly rotating under the pattern of coverage strips. The outline of continents and gridlines shown in FIG. 8 is just a snapshot of the Earth's position at a particular point in time. The width of a coverage strip, as depicted in FIG. 8, is about 10°. It takes the Earth less than forty minutes to rotate by 10°. Therefore, a location that is in the area of overlap at some particular time might easily no longer be in that area a just a few minutes later. Such erratic availability of enhanced capacity is generally regarded as not being very useful. On the other hand, if it were possible to guarantee that a particular location will be in an area of overlap at all times, it would be then possible to take full advantage of the double capacity. Alternatively, it would also be useful if it were possible, for example, to schedule in advance, and on demand, that a particular location will be in an area of overlap at a particular desired time in the future. In such a case, the extra capacity available in the area of overlap could be utilized effectively and advantageously.